How do I use thunderbird to use my iCloud email account

To access iCloud using a 3rd party app like Thunderbird, you need to turn on two-factor authentication for your Apple ID (if you haven't already) and use an app-specific password in Thunderbird rather than your usual Apple password. See:

Two-factor authentication for Apple ID - Apple Support

and

Using app-specific passwords - Apple Support

and

Mail server settings for iCloud email clients - Apple Support

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